It is related from some of the Salaf that backbiting, tale carrying and the likes break the fast, and it is mentioned as one opinion in the madhab of Imām Aḥmad. The final word on this issue is that Allāh the Exalted commanded people to fast in order to achieve piety (taqwā), and Allāh’s Messenger – praise and peace of Allāh be upon him – stated:

Whoever does not leave off false speech and acting by it; then Allāh is not in need of him abandoning his food and drink.

So if the fasting person does not achieve piety, he has not achieved what is intended through fasting, and so the reward of [his] fast will decrease in accordance [with how much he has gone against the intended goal, i.e. commensurate with his sins].

Righteous deeds have two intended goals: gaining reward and avoiding punishment. If a person fasts while also doing forbidden things, like backbiting, carrying tales between people or consuming what is forbidden and so on, he loses the reward.

Thus, when the imams say [backbiting etc.] does not break the fast, it means that the person who sins is not punished in the way a person who openly breaks his fast would be punished.

And those who said it does break the fast in the sense that the person has not achieved the intended goal behind fasting, or in the sense that he has lost the reward for fasting, then this statement is in agreement with the position of the imams.

One who says it breaks the fast in the sense that the person is to be punished for leaving [the fast], then he is in contradiction to the imams.

Conclusion

A person who disobeys Allāh while fasting hasn’t truly grasped the intent behind fasting. The real goal is to achieve piety and obedience of Allāh through the abandonment of food, drink and sin. Although a person who backbites, lies or does other sins is not considered to have physically broken his fast, he loses the reward of fasting and in this sense he has broken his fast.